© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2010, 22, 4190–4192 4190 www.advmat.de www.MaterialsViews.com COMMUNICATION wileyonlinelibrary.com By Hailing Guo, Yongzhong Zhu, Shilun Qiu,* Johannes A. Lercher,* and Hongjie Zhang* Coordination Modulation Induced Synthesis of Nanoscale Eu 1-x Tb x- Metal-Organic Frameworks for Luminescent Thin Films [∗] Dr. H. Guo, Prof. H. Zhang State Key Laboratory of Rare Earth Resource Utilization Changchun Institute of Applied Chemistry Chinese Academy of Sciences Changchun, 130022 (P. R. China) E-mail: hongjie@ciac.jl.cn Dr. Y. Zhu, Prof. J. A. Lercher Department Chemie Lichtenbergstrasse 4, 85748 (Germany) E-mail: johannes.lercher@ch.tum.de Prof. S. Qiu State Key Laboratory of Inorganic Synthesis and Preparative Chemistry College of Chemistry Jilin University Changchun 130012 (P. R. China) E-mail: sqiu@mail.jlu.edu.cn DOI: 10.1002/adma.201000844 Metal organic frameworks (MOFs) or porous coordination polymers (PCPs) are hybrid inorganic-organic materials made from an assembly of metal ions with organic linkers. [1–3] Their well-defined porosity and tunable chemical functionality make them extremely attractive for applications in gas storage, [4,5] catalysis, [6] and separation. [7] Apart from their use as bulk mate- rials, MOFs are also potential candidates for thin film appli- cations. Pioneering studies on MOF films or membranes have already been reported in the literature. [8–13] However, the reported approaches are either highly complex in preparation routes or ineffective in producing smooth and dense MOF thin films. [14] Therefore, developing a scalable method for MOF film preparation is definitely needed. Recently, lanthanide metal–organic frameworks (Ln-MOFs) have received special attention due to their unusual coordina- tion characteristics and exceptional optical and magnetic prop- erties arising from 4f electrons. [15–21] Their specific applications in thin film devices, however, depend greatly on the ability to control the size and shape of individual Ln-MOFs crystallites as well as their assembly on various supporting surfaces. Thus, it is very important to synthesize individual nanoscale Ln-MOFs crystallites that are useful for Ln-MOFs film preparation. To date, however, only a few approaches for the fabrication of nanometer-sized metal organic frameworks (NMOFs) are devel- oped. These include reverse microemulsion, [22,23] microwave- assisted synthesis [24] and the use of capping agents. [25,26] Here, we report an easily scalable method using carboxylate salts as capping reagent for the synthesis of nanosized Ln-MOFs crystals. We also show that these particles are well suited to make Eu 1-x Tb x -MOF films using the spin-coating deposition method. These films exhibited fascinating luminescence prop- erties and efficient Tb 3 + -to-Eu 3 + energy transferability. Our strategy for reducing the size of Ln-MOFs crystals to diameters of around 100 nm uses the addition of capping reagents with the same chemical functionality as the linkers. Typically, nanoscale Ln(BTC)(H 2 O), [19] hereafter denoted as Ln- MOFs, where Ln = Dy 3 + , Eu 3 + , or Tb 3 + and BTC = 1,3,5-benzen- etricarboxylate, were prepared by heating a solution containing LnNO 3 · xH 2 O (0.1 mol), BTC (0.1 mol), sodium carboxylate (0-0.3 mol), DMF (8 mL) and H 2 O (4 mL) in a sealed beaker at 60 ºC for 12-72 h. Sodium carboxylates (sodium formate, sodium acetate, or sodium oxalate) were used as capping rea- gent to control the resulting crystal size and morphology. After synthesis, the particles were isolated by centrifugation and washed several times with DMF and ethanol. The XRD diffrac- tion patterns of all samples agree well with literature suggesting phase purity and an unaffected MOF structure (see Supporting Information, Figure S1). Moreover, the diffraction peaks are sharper with addition of sodium formate and sodium acetate, implying that these two capping reagents can also improve the crystallinity of Ln-MOFs. Similar effects have also been observed by Kitagawa et al. [26] In the absence of a capping rea- gent, Ln-MOFs are pillar-like rods with a length of 60 ± 10 μm ( Figure 1a). With the addition of the capping reagent, both the shape and size of Ln-MOFs are drastically changed. Addition of sodium formate results in fairly uniform bean-shaped nanocrys- tals with a length and width of 125 ± 25 nm and 100 ± 15 nm, respectively (Figure 1c, d). Smaller crystals, 90 ± 15 nm in length and 75 ± 10 nm in width, are obtained, when sodium acetate is used as additive (Figure 1e, f). However, sodium oxalate, leads to the needle-shaped crystals (Figure 1b) and the length of the crystals with lengths between 30–60 μm. The role of carboxylic salts in reducing the particle size is attributed to its modulating effect on the coordinating interac- tions between the metal ions and organic linkers. In the initial stage of synthesis, Ln cations coordinate to carboxylic groups, which are not only from organic linker BTC but also from car- boxylic salt added. The crystal growth is, therefore, impeded in a very early stage allowing more nuclei to be formed. Moreover, the competitive coordination of the capping reagent is specu- lated to regulate the rate of crystal growth. [26] In the case of sodium acetate and sodium formate, their appropriate inter- actions with Ln cations slow down the rate of crystal growth leading to smaller and relatively uniform nanoparticles.